1. Field of the Invention
This invention relates generally to drilling jars and, in particular, to a double-acting mechanical-hydraulic drilling jar.
2. Description of the Related Art
Drilling jars have long been known in the field of well drilling equipment. A drilling jar is a tool employed when either drilling or production equipment has become stuck to such a degree that it cannot be readily dislodged from the well bore. The drilling jar is normally placed in the pipe string in the region of the stuck object and allows an operator at the surface to deliver a series of impact blows to the drill string via a manipulation of the drill string. These impact blows to the drill string are intended to dislodge the stuck object and permit continued operation.
Drilling jars contain a sliding joint which allows a relative axial movement between an inner mandrel and an outer housing without allowing relative rotational movement therebetween. The mandrel typically has a hammer formed thereon, while the housing includes an anvil positioned adjacent to the mandrel hammer. Thus, by sliding the hammer and anvil together at high velocity, a substantial jarring force may be imparted to the stuck drill string, which is often sufficient to jar the drill string free. For most fishing applications it is desirable that the drilling jar be capable of providing both an upward and a downward jarring force.
There are four basic forms of drilling jars: purely hydraulic jars, purely mechanical jars, bumper jars, and mechanical-hydraulic jars. The bumper jar is used primarily to provide a downward jarring force. The bumper jar ordinarily contains a splined joint with sufficient axial travel to allow the pipe to be lifted and dropped, causing the impact surfaces inside the bumper jar to come together to deliver a downward jarring force to the string.
Mechanical, hydraulic, and mechanical-hydraulic jars differ from the bumper jar in that they contain some type of tripping mechanism which retards the motion of the impact surfaces relative to each other until an axial strain, either tensile or compressive, has been applied to the drill string pipe. To provide an upward jarring force, the drill pipe is stretched by an axial tensile load applied at the surface. This tensile force is resisted by the tripping mechanism of the jar long enough to allow the pipe to stretch and store potential energy. When the jar trips, this stored energy is converted to kinetic energy causing the impact surfaces of the jar to move together at a high velocity. To provide a downward jarring force, the pipe weight is slacked off at the surface and, if necessary, additional compressive force is applied, to put the pipe in compression. This compressive force is resisted by the tripping mechanism of the jar to allow the pipe to compress and store potential energy. When the jar trips, the potential energy of the pipe compression and pipe weight is converted to kinetic energy causing the impact surfaces of the jar to come together at a high velocity.
The tripping mechanism in most mechanical jars consists of some type of friction sleeve coupled to the mandrel which resists movement of the mandrel until the load on the mandrel exceeds a preselected amount (i.e., the tripping load). The tripping mechanism in most hydraulic jars consists of one or more pistons which pressurize fluid in a chamber in response to movement by the mandrel. The compressed fluid resists movement of the mandrel. The pressurized fluid is ordinarily allowed to bleed off at a preselected rate. As the fluid bleeds off, the piston translates, eventually reaching a point in the jar where the chamber seal is opened, and the compressed fluid is allowed to rush out, freeing the mandrel to move rapidly.
Mechanical jars and hydraulic jars each have certain advantages over the other. Mechanical drilling jars are generally less versatile and reliable than hydraulic drilling jars. Many mechanical drilling jars require the tripping load to be selected and preset at the surface to trip at one specific load after the drilling jar is inserted into the well bore. If it is necessary to re-adjust the tripping load, the drilling jar must be pulled from the well bore. Other mechanical jars require a torque to be applied to the drill string from the surface in order to trigger the jar. The applied torque to the drill string not only represents a hazard to rig personnel, but torque cannot be applied to coiled tubing drill strings. Another significant disadvantage of mechanical jars is apparent in circumstances where the jar must be placed in a cocked position prior to insertion into the well bore. Thus, in those circumstances, the tripping mechanism is subjected to stresses during the normal course of drilling if the jar is run as part of the bottom hole assembly. Finally, many mechanical jars have many surfaces that are subject to wear.
Hydraulic drilling jars offer several advantages over purely mechanical drilling jars. Hydraulic drilling jars have the significant advantage of offering a wide variety of possible triggering loads. In the typical double acting hydraulic drilling jar, the range of possible triggering loads is a function of the amount of axial strain applied by stretching or compressing the drill pipe, and is limited only by the structural limits of the jar and the seals therein. In addition, hydraulic drilling jars are ordinarily less susceptible to wear and, therefore, will ordinarily function longer than a mechanical jar under the same operating conditions. However, hydraulic drilling jars also have certain disadvantages. For example, most purely hydraulic double acting drilling jars are relatively long, in some instances having a length exceeding 25 feet. The length of a particular jar is ordinarily not a significant issue in drilling situations where regular threaded drill pipe is utilized. However, in coiled tubing applications, it is desirable that the length of all the tools in a particular drill string be no longer than the length of the lubricator of the particular coiled tubing injector. Thus, it is desirable that the drilling jar be as short as possible to enable the operator to place as many different types of tools in the drill string as possible while still keeping the overall length of the drill string less than the length of the lubricator. A conventional hydraulic drilling jar may take up one-half or more of the total length of a given lubricator, thus leaving perhaps less than half the length of the lubricator to accommodate other tools such as a mud motor, an orienting device, or a logging tool.
Many hydraulic drilling jar designs also have a disadvantageously long metering stroke. The metering stroke is the amount of relative movement between the mandrel and the housing that must occur for the jar to trigger after it is cocked by application of an axial load. When an ordinary hydraulic drilling jar is cocked by application of an axial load, fluid is pressurized in a chamber to resist relative movement of the mandrel and the housing. One or more metering orifices in the jar allow the compressed fluid to bleed off at a relatively slow rate. As the fluid is bleeding off, there is some relative axial movement between the mandrel and the housing. The amount of relative axial movement between the mandrel and the housing that occurs after the jar is cocked, but before the jar triggers, is known as bleed off. The bleed off represents lost potential energy that would ordinarily be convened into additional jarring force. Many current hydraulic drilling jar designs have a relatively long metering stroke of 12 inches or more and, therefore, a significant amount of bleed off. A long metering stroke also leads to heat buildup in the hydraulic fluid, which may require costly intervals between firings and lead to degradation of fluid.
Mechanical-hydraulic drilling jars ordinarily combine some features of both purely mechanical and purely hydraulic drilling jars. For example, one design utilizes both a slowly metered fluid and a mechanical spring element to resist relative axial movement of the mandrel and the housing. This design has the same disadvantages associated with ordinary hydraulic drilling jars, namely length, long metering stroke, and fluid heating. Another design utilizes a combination of a slowly metered fluid and a mechanical brake to retard the relative movement between the mandrel and the housing. In this design, drilling mud is used as the hydraulic medium. Therefore, the string must be pressurized before the drilling jar will operate. This pressurization step will ordinarily require a work stoppage and the insertion of a ball into the work string to act as a sealing device. After the drilling jar is triggered, the ball must be retrieved before normal operations can continue.
The present invention is intended to overcome or minimize one or more of the foregoing disadvantages.